The term “patterning device” as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such a patterning device include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident radiation as diffracted radiation, whereas unaddressed areas reflect incident radiation as undiffracted radiation. Using an appropriate filter, the said undiffracted radiation can be filtered out of the reflected beam, leaving only the diffracted radiation behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at one time; such an apparatus is commonly referred to as a wafer stepper or step-and-repeat apparatus. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic apparatus as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling an incoming radiation beam, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCT patent application WO 98/40791, both incorporated herein by reference.
Developments in the field of lithography for improving projection image quality have demonstrated the necessity of improving several characteristics of the radiation beam as provided in a lithographic apparatus. Generally, these characteristics comprise the homogeneity or uniformity of the beam, the size and/or shape of the beam, the pointing direction of the beam, and the divergence of the beam, further indicated as “beam quality characteristics”. Until now, only limited options are available in a lithographic system to determine such beam quality characteristics and to control such characteristics. Specifically, when a radiation beam lacks a proper dimensioning and homogeneity, the ellipticity, for example, of the projection beam can be affected, causing a different imaging quality of horizontal and vertical structures on a substrate.
A lithographic system typically uses a radiation source, such as for example a laser, which provides a radiation beam that is further lead into an illumination system and subsequently towards a patterning device, such as a mask. Such a radiation source is a unit which is generally positioned at a certain distance from the rest of the lithographic apparatus, which typically comprises the illumination system, the patterning device support structure, the projection system, and the substrate table.
The illumination system is used to guide the beam towards a support structure for supporting a patterning device and to control the illumination thereof. Hence, in a conventional lithographic system, the beam travels from the radiation source to the illumination system, and, before entering the illumination system, simple conditioning of the beam is possible, such as manually adjusting an optical conditioning unit such as a beam expander in order to modify the size of the beam in two different directions or directing the beam by one or more steering mirrors.
In order to determine the beam size of a radiation beam traveling through the illumination system, conventionally, a fluorescent raster target is placed in the optical path of the beam and the dimensions of the beam are visually interpreted or graphically recorded by, for example, a CCD camera. An example of such a camera arrangement is given in U.S. Pat. No. 4,916,319. The image from the camera may then be analyzed further. Based on the value of the beam size thus determined, the optical conditioning unit can be manually adjusted. This process is repeated until a proper beam size is achieved, so that the beam is tuned—until a new adjustment of the lithographic system appears necessary.
In order to determine the beam divergence, conventionally, the fluorescent raster target is placed in at least two positions along the optical path of the beam, the difference in beam size between the two positions being indicative of the divergence of the beam. Although considered relevant for achieving optimal illumination conditions, no quick or automated procedure exists in order to measure a beam divergence of the beam, which although small, may be able to influence the system in quite a severe manner, since in a lithographic system the imaging pupil will be distorted depending on such beam quality characteristics.
It is clear that the above procedures for determining beam size and divergence are tedious and time-consuming, let alone the costs involved of keeping the apparatus in a non-productive state during such procedures.
In a number of lithographic apparatus, a detector for detecting the intensity of the radiation beam in the illumination system is present. Such a detector is for example an energy sensor (ES), which captures a split version of the beam and measures an energy influx, in order to provide a control of the radiation dose delivered to a target portion of the substrate. Another example of such a detector is a positional detector used in a so called beam measuring unit (BMU), which is able to determine the position and pointing direction of the radiation beam in the illumination system. The BMU comprises focusing elements that image split versions of the beam on two position sensitive devices (PSDs), the position on the first PSD being indicative of the entering position of the beam and the position on the second PSD being indicative of the pointing direction of the beam. A PSD can operate as a detector for measuring a radiation intensity when the output signals of the PSD are being summed or integrated.